This invention relates to a process for separating and recovering Kr-85 from a waste gas generated in the reprocessing of used nuclear fuel by cryogenic distillation, and more particularly to a process for separating and recovering Kr-85 by removing explosive gaseous substances, or materials capable of forming explosive gaseous substances or condensible substances, which may clog a piping system of a cryogenic distillation apparatus from the waste gas, and then leading the gas to the cryogenic distillation apparatus: In other words, the present invention concerns a process for treating a waste gas from the reprocessing of nuclear fuel with an increased safety of operation of the cryogenic distillation apparatus used for the separation and recovery of Kr-85.
It is very important from the viewpoint of effective utilization and reduction in operating cost of nuclear facility to recover and reuse the effective components contained therein after burning the nuclear fuel to some extent. In the reprocessing of the used nuclear fuel destined to the reuse of the nuclear fuel, clad tubes of fuel elements are dismantled, and the nuclear fuel is taken out of the fuel elements and dissolved in a nitric acid solution, while blowing oxygen or air into the solution. Then, oxygen or air is further blown into the resulting nitrate solution of the nuclear fuel to precipitate the impurities, and then the solution is transferred to a step for recovering the effective components.
As the dissolution and precipitation treatments, radio-active, gaseous substances including Kr-85 (the substances will be hereinafter referred to as Kr-85) are evolved from the nuclear fuel. Furthermore, a considerable amount, for example, several hundred ppm, of nitrogen oxides, NOx, is generated owing to the use of the nitric acid solution. Furthermore, hydrocarbons are contained therein.
Since the waste gas containing Kr-85 cannot be discharged to the atmosphere as such, it is necessary to separate and recover Kr-85 from the waste gas, keep it stored for a half-value period of Kr-85 to make Kr-85 harmless, and then dispose it. A very large amount of the waste gas is evolved from the reprocessing plant of the nuclear fuel, but as a result of studies, the present inventors have already confirmed that a cryogenic distillation process is most suitable for treating such a large amount of the gas. On the basis of the finding, two of the present inventors developed several processes for separating and recovering Kr-85 from the waste gas by the cryogenic distillation process, one of which is disclosed in H. Yusa et al U.S. patent application Ser. No. 335,749, with a title "Process and apparatus for separating and recovering krypton-85 from exhaust gas of nuclear reactor or the like", filed on Feb. 26, 1973.
In the treatment of the waste gas in the cryogenic distillation apparatus, it is necessary to remove condensible substances such as carbon dioxide, water, etc. in advance, because there is a danger of clogging the piping system of the cryogenic distillation apparatus with these condensible substances. These condensible substances can be removed from the waste gas by dehumidification, adsorption by active carbon, condensation-separation, or absorption by acid or alkali. As a result of further study on the process for separating and recovering Kr-85 by the cryogenic distillation method, the present inventors have confirmed that the hydrocarbons, nitrogen oxides, and oxygen contained in the waste gas are hazardous substances to the process for separating and recovering Kr-85. That is to say, hydrocarbons and nitrogen oxides have a possibility of explosion within the cryogenic distillation apparatus, and further oxygen can produce explosive substances, that is, ozone and nitrogen oxides, through action of radioactive rays. Furthermore, in view of the current situations that control on the industrial wastes are severer year by year, it is not preferable to discharge a waste gas containing a large amount of nitrogen oxides, which are regarded as origins of photochemical smogs, to the atmosphere as such. Therefore, it is desirable to make the nitrogen oxides harmless within the waste gas-treating plant.